If you are coming into Yellowstone National Park from the South Entrance Road (also known as Highway 89) be prepared to stop minutes before Lewis Lake to hike to this waterfall. After crossing the bridge over the Lewis River pull over on the right hand side of the road, there should be plenty of parking. You will walk back towards the bridge and though you can see the Lewis Falls from the road I would recommend getting a closer look, especially if you intend on photographing this little gem. 

The trail is unmarked and there are various little paths but all of them ultimately lead to the same location. The trail will be located on the east side of the river banks and go towards the falls. The elevation gain is minimal but can get steep at certain points. After about five minutes you should see Lewis Falls right in front of you and proceed to carefully walk down the hill to the shore where you can photograph until your heart is content. 

Note that there are other possible unmarked trails on the opposite side of the river but this one is best for direct sight of the falls especially for photographing.

Also note there are possibilities of seeing wildlife here so be mindful and respect their space.